1. Field of Invention
The invention relates to a voltage conversion system which converts voltage using a converter, a voltage conversion method thereof, and a recording medium that stores a program for implementing the voltage conversion method.
2. Description of Related Art
An inverter has generally been used for driving an alternating current (AC) motor such as a permanent magnet motor. More specifically, direct current supplied from a battery is converted into a desired form of alternating current by means of an inverter and thereafter is applied to a motor to drive it. Especially, in an electric motor vehicle or a hybrid motor vehicle, it is necessary to finely control the output of the motor, therefore such a system using an inverter is preferably used.
In a case that a motor is driven by use of a system including an inverter as described above, however, when an input voltage of the inverter is low, it may cause an undesirable state where current necessarily becomes high to achieve a high output of the motor. In view of this, there is a demand for maintaining the input voltage of the inverter sufficiently high. On the other hand, a battery is basically constituted of battery cells each having output voltage of approximately 1V. For obtaining a high battery voltage, therefore, it is necessary to connect many battery cells in series. To avoid this, it is demanded, on the contrary to the above demand, that the battery voltage is made as low as possible.
In view of the above situation, it has been proposed to increase a battery voltage by means of a boost converter and thereafter input it to an inverter. With this arrangement, it is possible to set a high inverter input voltage even if the available battery voltage is low.
FIG. 6 shows one example of such a conventional motor drive circuit including a converter. A positive terminal of a battery 10 is connected to a converter 12 that includes a coil L and transistors Q1, Q2. One end of the coil L is connected to the positive terminal of the battery 10. An emitter of the transistor Q1 is connected to the other end of the coil L while a collector thereof is connected to a positive output line of the converter 12 (a positive bus bar of an inverter), and a collector of the transistor Q2 is connected to the same end of the coil L and the emitter of the transistor Q1 while an emitter thereof is connected to a negative terminal of the battery 10 (a negative output line of the converter 12 connected to a negative bus-bar of the inverter). Further, diodes D1, D2 are respectively connected between the emitter and the collector of the transistors Q1, Q2, so as to allow the current to flow therethrough only in one direction from the emitter side to the collector side.
The transistors Q1, Q2 are switched on/off alternately to change an “ON” time ratio therebetween as needed for achieving a desired high output voltage of the converter 12.
Besides, a smoothing capacitor C is arranged between the positive and negative output lines of the converter 12 so as to smooth the output of the converter 12.
The positive and negative outputs of the converter 12 smoothed by the capacitor C are respectively input to the positive and negative bus bars of the inverter 14. The inverter 14 includes six transistors Q3 to Q8 and is adapted to produce three different phase outputs. More specifically, the transistors Q3 and Q4, the transistors Q5 and Q6, and the transistors Q7 and Q8 are respectively connected to each other in series between the positive and negative bas bars, thus forming three phase arms. Each connecting point between the transistor located in the upper side of each phase arm, namely the transistor Q3, Q5, or Q7, and that located in the lower side thereof, namely the transistor Q4, Q6, or Q8, provides each phase output of the inverter 14. Also, diodes D3 to D8 are respectively connected between the emitter and the collector of the transistors Q3 to Q8 so as to allow the current to flow therethrough only in one direction from the emitter side to the collector side.
Each of the three phase outputs of the inverter 14 is connected to one end of a corresponding one of phase coils of a three-phase AC motor 16 (hereinafter will be simply referred to as “motor 16”).
With the motor drive circuit constructed as described above, when driving the motor 16, necessary one or ones of the transistors Q3 to Q8 are switched on such that the transistors in the upper side of the respective phase arms and the transistors in the lower side thereof are not ON at the same time, thus applying three phase currents shifted by 120° from one another to the motor 16.
In this circuit, there also provided voltage sensors 20a, 20b, 22a and 22b, and current sensors 24a, 24b and 24c. The voltage sensors 20a, 20c are both used for detecting the voltage of the battery 10 (battery voltage: converter input voltage) while the voltage sensors 22a, 22b are both used for detecting the voltage of the capacitor C (converter output voltage: inverter input voltage). The current sensors 24a, 24b, and 24c are used for detecting the respective phase currents applied to the motor 16. The detected values of these sensors and command values for controlling the motor output are input to the control unit 26. In accordance with these values, the control unit 26 switches on/off the transistor Q1 in the upper side of the converter 12 and the transistor Q2 in the lower side thereof so as to obtain a desired output voltage of the converter 12, while switching on/off the transistors Q3 to Q8 of the inverter 14 so as to bring the output of the motor 16 to a motor output command value.
The operations of the converter 12 and the inverter 14 are both controlled using a so-called PWM (Pulse Width Modulation) control. More specifically, a desired voltage command value is set with respect to a predetermined triangular carrier (wave), and the duty ratio between the transistors Qi, Q2 is adjusted to control the voltage conversion (i.e. voltage increase rate or voltage decrease rate).
On the other hand, when controlling the output of the motor 16, the transistors Q3 to Q8 of the inverter 14 are switched on/off according to a result of a comparison between a voltage command value for the phase outputs and the predetermined triangular carrier (wave), so as to achieve the voltage command value.
In the motor drive circuit shown in FIG. 6, as described above, there also provided two voltage sensors 20a, 20b for detecting the voltage of the battery 10 and another two voltage sensors 22a, 22b for detecting the voltage of the capacitor C. This is because it is necessary to detect the input and output voltages of the converter 12 and to detect the input voltage of the inverter 14 for controlling their operations. With the two voltage sensors (20a and 20b, or 22a and 22b) provided in each location, further, the voltage can be reliably detected even in the event of a failure of each voltage sensor.
More specifically, having two voltage sensors in each location as above makes it possible to detect the voltage even when one of the sensors fails, and thus provides the fail-safety of the system. However, such arrangement involves a problem that the overall cost of the system becomes high since four sensors are needed. Also, such arrangement may further cause the following problems. That is, the converter 12 becomes uncontrollable when the voltage sensors 20a, 20b for detecting the voltage of the battery 10 both fail, and the converter 12 and the inverter 14 both become uncontrollable when the voltage sensors 22a, 22b for detecting the voltage of the capacitor C both fail.